Microwave and millimeter-wave technology has application in a variety of areas, such as in satellite or terrestrial communication, radar, and astronomy. Many of these applications use polarized radiation in their operation. The polarization may be circular or linear, and some systems use both types of polarization or convert from one type to the other. Other systems may require that the radiation is converted between linear, left-circular, and right-circular polarizations or that the phase or polarization state of the radiation is varied continuously. The conversion typically takes place within a waveguide, and the components that perform the conversions are generally termed “phase shifters,” “circular polarizers,” “phase retarders,” or simply “retarders” in the art.
An example of a conversion in practice is the rotation of the orientation of linearly polarized microwave radiation in satellite communications. Some satellite microwave antennae are linearly polarized. Moving the satellite to a different orbit or communicating with a different ground station may require that the orientation of linear polarization be changed. One method of accomplishing the reorientation is by converting the linearly polarized radiation to circularly polarized radiation, and then converting the resulting circularly polarized radiation back into linearly polarized radiation but with the changed orientation. Such a change may be accomplished by one or more retarders within the waveguide that feed the antenna of the satellite or the antenna of the ground station.
Alternatively, some communication antennae are circularly polarized, and the communication does not require matching of the orientation of the transmitter and the receiver. Such systems, however, may include a linearly polarized transmitter or receiver. Coupling a circularly polarized antenna to the transmitter or receiver may be accomplished by one or more retarders within the waveguide that connects the antenna to the transmitter or receiver.
A retarder has two orthogonal principal axes. Radiation that is linearly polarized along one principal axis receives a phase shift with respect to radiation that is linearly polarized along the other principal axis. As is known in the art, converting linearly polarized radiation to circularly polarized radiation may be accomplished by a retarder whose principal axes are oriented at 45° to the linearly polarized radiation and which imposes a phase shift of 90° with respect to the orthogonal polarization states. This configuration of the retarder is called a quarter wave retarder or a circular polarizer. In general, by selecting different orientations with respect to incident radiation and by designing the retarders to impose different phase shifts, components with a variety of properties are possible.
It is generally desired that retarders operate efficiently and precisely over a broad range of frequencies. As is known in the art, there are many convenient parameters that may be used to measure the efficiency or precision of the retarder. For example, a retarder configured as a circular polarizer may efficiently convert linearly polarized radiation to circularly polarized radiation within its bandwidth, but produce polarized radiation that is unacceptably elliptical at frequencies that lie outside the bandwidth. One measure of the efficiency of a circular polarizer is known as the axial ratio in the art. In the case of a right-handed circular polarizer, inefficient operation results in a leakage of radiation that is left-handed polarized. The leakage of the right-handed circular polarizer may be defined as the complex voltage amplitude, DR, of the left-handed circular response of the polarizer. In the case where linearly polarized radiation is received by the retarder, DR is the voltage corresponding to the components of the electric field of the left-handed polarized radiation that is transmitted by the polarizer. The axial ratio, A, may then be defined by equation Eq. 1:                     A        =                  20          ⁢                                           ⁢                                    log              10                        ⁡                          [                                                                                          1                      -                                                                                                                              D                            R                                                                                                    2                                                                              +                                                                                D                      R                                                                                                                                                      1                      -                                                                                                                              D                            R                                                                                                    2                                                                              -                                                                                D                      R                                                                                                      ]                                                          (                  Eq          .                                           ⁢          1                )            An axial ratio of zero decibels (“dB”) corresponds to a perfect polarizer with no leakage into the orthogonal polarization state. The frequency range over which the axial ratio is below a certain level, divided by the center frequency, can be used to define the bandwidth of the polarizer. The bandwidth may also be expressed as a percentage, by dividing the frequency range by the center frequency.
Methods for constructing waveguide retarders include incorporating corrugations or ridges on the inside walls of the waveguide, or introducing dielectric slabs within the waveguide. Variations on these structures have been constructed in an attempt to achieve a large bandwidth.
One example of a waveguide retarder is disclosed in Lier, E. and Schaugg-Pettersen, T., A Novel Type of Waveguide Polarizer with Large Cross-Polar Bandwidth. IEEE Transactions in Microwave Theory and Techniques, vol. 37, no. 11, pp. 1531-1534 (1988). The paper discloses a single element circular polarizer constructed by incorporating transverse corrugations into the walls of the rectangular waveguide. In this configuration, an axial ratio of less than 0.11 dB is achieved over a bandwidth of approximately 28%.
Another example of a waveguide retarder is disclosed in Uher, J., Bornemann, J., and Rosenberg, U., Waveguide Components for Antenna Feed Systems: Theory and CAD, pp. 419-433, Boston, Artech House, 1993. The book discloses single element circular polarizers including those constructed by tapering the waveguide, incorporating corrugations into the walls of the waveguide, and introducing dielectric slabs into the waveguide. In these configurations, bandwidths of up to approximately 40% with an axial ratio less than 0.37 dB may be achieved.
A further example of a waveguide retarder is disclosed in the U.S. Pat. No. 6,097,264 to Vezmar. The patent discloses a single element circular polarizer incorporating four axial ridges into the walls of the waveguide. In these configurations, bandwidths of up to approximately 60% may be achieved, but with relatively high leakage indicated by an axial ratio of less than 1.7 dB.
For many applications, however, larger bandwidths or lower leakages are desired. Therefore there is a need for a retarder or polarizer that has little leakage over a broad bandwidth.